Tuning forks and oscillators embodying the same

ABSTRACT

A tuning fork assembly fabricated from a flat strip of material with the tines being formed by the removal of a portion of the strip so that the longitudinal axis of each tine lies in a plane which is substantially orthogonal to the directions in which the tines move during normal operation of the fork. A compliant base portion readily responds to and actuates drive and pickup transducers and for the purpose of reducing or cancelling spurious signals in the pickup transducer caused by unwanted ambient vibrational forces acting upon the fork, an auxiliary pickup is electrically connected in out-of-phase relationship to the main pickup.

United States Patent Reefman [4 1 Jan. 25, 1972 [S4] TUNING FORKS AND OSCILLATORS FOREIGN PATENTS 0R APPLICATIONS EMBODYING THE SAME 444,763 2/1968 Switzerland ..5s 23 TF [72] Inventor: Wlllilm E. Reefman, Santa Barbara, Calif. Primary Examiner-mchard B Wilkinson t [73] Assignee: The Bunker-Ramo Corporation. Oak Assistant Examiner-Lawrence R. Franklin Brook, Ill. Attorney-Frederick M. Arbuckle [22] Filed: July 23, 1969 l 57] ABSTRACT [21] Appl' 843923 A tuning fork assembly fabricated from a flat strip of material with the tines being formed by the removal of a portion of the [52]. 0.8. CI. ...84/4s7, 58/23 TF strip so that the ng in l axis of each in i in a pl ne [51 Int. Cl. ..Gl0g 7/02 which is substantially orthogonal to the directions in which the [58] Field of Search ..84/457; 58/23 TF; 310/25 tin move during normal operation of the fork. A compliant base portion readily responds to and actuates drive and pickup [56] Reference Cited transducers and for the purpose of reducing or cancelling spurious signals in the pickup transducer caused by unwanted am- UNITED TATES PATENTS bient vibrational forces acting upon the fork, an auxiliary 2,601,801 7/1952 Langloys ..84/457 i if 3,462,939 8/1969 Tanaka et al ..5s 23 TF 8 Claims, 12 Drawing Figures mzmwmsmz 31636310 Hem FIGSB INVENTOR WILLIAM E, REEFMAN PATENTEU JANZS I972 sum 2 or 2 lNVEHTOR 1:- WILLIAM EREEFMAN ment thereof so as to provide a stable source of continuous electrical signals at some predetermined signal frequency.

The present invention is further directed to a methodof fabricating and the structural characteristics of low-cost-massproduceable high Q (low loss) .tuningforks which compared to prior art tuning forks are light in weight and dimensionally small yet capable of being driven with very low power requirements into temperature stable vibrations at relatively low frequencies such as 20 to 20,000 Hertz.

The present invention is additionally directed to methods and structure for e'lectromagnetically and piezoelectrically producing, maintaining and sensing the vibrations of tuning forks having the aforesaid characteristics in a manner so as to minimize the affect upon the vibration thereof, of spurious ambient vibrational forces that may be imposed upon such forks.

In many electronic .and electromechanical systems and devices there is a need for a highly stable source of'electrical signalsat some predetermined frequency. In some instances the desired frequency is relatively low and it becomes difficult to produce a low cost highly reliable all electronic oscillator circuit which has sufl'lciently long term stability over a wide range of ambient temperature arid power supply voltage variations as to form aniaccurate driving signaltime base, for example, suitable for powering a signal frequency'dependent-e'lectromechanical clock or the like. It has been shown, however, that temperature compensated tuning forks having a sufficiently high Q (low loss) can be employed as the frequency controlling element in the feedback circuit of an electronic oscillator circuit so as to provide a suitable long term time base for driving such electric clocks. Notwithstanding, at the usually required clock drive frequencies ranging from say 60 to 500 Hertz conventional tuning forks become quite heavy and bulky making it impractical to consider packaging such a fork along with the necessary electronics; motor, dial assembly and gear mechanisms which are employed in compact clocks such as, for example, those designed for use in automobiles. Furthermore, because of the spurious vibrational forces which are typically imposed upon a clock mounted in an automobile a conventional tuning fork if packaged with such a clock would be in turn subject to these spurious vibrations and thereforetend to assume vibrational modes which interfere with its normal functioning as the frequency controlling element in its oscillatory drive circuit.

Moreover, it is well known that conventional low-frequency tuning forks (sometimes referred to as fork-mode vibrators) are generally formed by bending a ribbon of low molecular loss metal into a U-shaped configuration and then affixing a I support member thereto at the base of the U in alignment with the longitudinal axis of revolution thereof and extending outwardly away from the base of the U. Alternatively, the fork may be milled from a rectangular block of material into a U- shaped' configuration having a relatively heavy section at the base of the U. Each of the legs of the U extending from the base act as a tine" of the fork and upon being struck or otherwise mechanically displaced will be set into vibration at some predetermined frequency. The time, care and cost of the low loss materials required in making such forks militate against their adoption for use in driving signal time bases for low-cost mass-produced signal frequency dependent clocks;

In accordance with the present invention the principal disadvantages of prior art approaches taken in the design and manufacture of tuning forks are overcome and when taken in combination with the fork driving, and sensing methods and structures embraced by the present invention the improved tuning fork contemplated thereby becomes the basis of a practical low-cost mass-produced highly reliable and highly stable driving signal time base for self-contained signal frequency dependent electromechanical devices.

5 either or both the reed member and U-member are mechani- More particularly, the fpresent invention contemplates a tuning fork assemblywhich is characterizedby having at least two tines the longitudinal axisof each of which liessubstantially in a common plane which is in turn orthogonally disposed to line vectors generally indicating the path and directions of tine vibration in what willbe hereinafter-more clearly understood as-the fork-mode. Accordingly, atuning forkdn aceordance'with'thepresent invention may be massproduced by simply stamping out a portion of aflat 'thin rectangularstrip of material to format least two'tineme'nibers extendingfrom arectangular base section.

, I In a preferred form of the'invention thetines-areformed by the removal of a relatively narrow U-sh'aped portion of "the strip withthebase of the U-shaped aperture thus formed in the strip being adjacentone edged the strip. Theresulting configuration' of the strip therefor comprises arectangular base member from which there extends for vibration relative thereto=a first rectangular reed member surrounded by a U:

shaped mem'ber'with the'outer extremities of the two leg sections of the U-member joined to the base section along the same line as is the reed member. Preferably the-aggregateof the widths of'th'e-two leg sections of the -U-membe'r should .be

substantially equal to the width of the rectangular reed member. In accordance with the present invention when cally displaced and sharply released both members assume a mode of vibration corresponding to the tines of a tuning'fork.

Still further in accordance with "the present invention by properly .positioningvibrational drive and sense means rela tive to'or upon a tuning fork of the type contemplated by the present'invention the otherwise deleterious affects of spurious ambient vibrations upon the frequency determining role which the fork may play in an oscillatory circuit are effectively overcome.

'Abetter understanding of the present invention and its features of advantage will be gained by a reading of the following specification and'drawings in which:

FIGS. l and 2 illustrate in perspective conventional tuning fork constructions; 7

FIG. 3 illustrates one form of a flat'tuning fork configured in accordance with the principles of the present invention;

FIGS. 4A and 4B illustrate the fork-mode vibration of the tuning fork illustrated in FIG. 3; I,

FIGS. 5A, 5B and 5C illustrate, respectively, different configurations of tuning forks made in accordance with the present invention and how the resonant frequencies of the two tines thereof which are of different length may be made the same;

FIG. 6 is a perspective, diagrammatic and schematic illustration of structure contemplated by the present invention for continuous vibration of tuning forks made in accordance with the present invention and for cancelling certain undesirable spurious ambient vibrational influences thereon;

FIGS. 7 andfl illustrate another embodiment of the present invention by whichspurious vibrational effects may be cancelled;

FIG. 9 illustrates still another form of tuning fork made in accordance with the present invention as well as a preferred arrangement for effecting cancellation of spurious vibrational effects.

Turning now to FIGS. 1 and 2, there is shown two different forms of typical prior art tuning forks. The fork of FIG. 1 is of a type which is generally fabricated from a rectangular block oflow molecular loss material such as Ni Span G, atwo-phase alloy made up of chromium and titanium, which after milling in the shape shown is heat treated. The tines 4' and 4 of the tuning fork are seen to extend in substantially parallel relationship to one another from the base 8 with the curved portion of the base 6extending betweenthetines being generally ref'erredto as the throat of the tuning fork.

The prior art tuning fork of FIG. 2 on the other hand, is-

throat section 6a at the lower outer extremity of which is affixed a cylindrical base member 8a.

Before proceeding further with a description of the present invention, consideration will first be given to some of the operational characteristics of tuning forks in general. In any tuning fork it is of course highly desirable to obtain a high effective Q of vibration, that is, the energy stored in the tunin g fork system during any given cycle of vibration is high with respect to the energy which is dissipated during that cycle.

There are several factors which will influence the Q of a tuning fork all of which are based upon the mechanical dynamics involved at the throat, base and mounting of the fork. Normally, the tines of a vibrating tuning fork are at any given instant moving in opposite directions with respect to one another which, based upon the law of conservation of energy, will minimize the vibrational forces transmitted to the base. Thus, as shown in FIGS. 1 and 2 when the tines 4 and 4' are moving the direction of arrows 5a, the tines 4 and 4a are moving in the direction of arrows 51;. Similarly, when the tines 4 and 40 move in the direction of arrows 7a the tines 4' and 4a move in the direction of arrow 7b. This is called normal forkmode" vibration.

Fork-mode vibration is caused by reason of the fact that two tines of each fork are mechanically coupled to one another at their base extremities by the metal molecules in or near the throats 6 and 6a of the forks. The degree of coupling between the two tines determines the extent to which the two tines will vibrate in exact synchronism, the greater the mechanical coupling the greater the synchronism between the two tines. If an unbalance in the self-resonant frequency exists between the two individual tines of a fork, that is, one tine taken by itself is resonant about the base at a slightly different frequency than the other tine taken by itself and if the coupling between the two tines is close to unity, the frequency of the tuning fork resonance will be the average of the self-resonant frequencies of the two tines.

In instances where the two tines are not tuned to exactly the same frequency so called unbalance energy" will be developed within the molecules in and around the throat area and vibrationally transmitted to the base. If the material from which the tuning fork is made is characterized by having a high molecular frictional loss, this unbalance energy causing vibration of the metal molecules at the resonant frequency of the fork will be dissipated in heat and thus reduce the Q of the fork. Additionally, the principal work done by unbalance energy transmitted to the base of a tuning fork generally takes place in and around where the fork is mounted to some supporting means such as between wood or plastic blocks or the like (not shown). It is therefore desirable in any tuning fork to provide tight coupling between the tines to ensure a high degree of synchronism between the tines and to minimize the dissipation of unbalance energy by balancing the self-resonant frequencies of the tines.

Going back now to the prior art tuning fork shown in FIG. 1 and 2, it can be seen that the relative coupling between the tines 4 and 4a which depends upon what may be called the stress/strain communicating ability of the material forming the throat 6 will be relatively low since these forces must pass through the relatively massive base 8 which expresses considerable inertia.

Also, on the other hand, it will be seen that the prior artfork shown in FIG. 2 has a relatively high coupling between the tines 4a and 4a since the entire throat area 6a in effect forms a part of the tines of themselves. This provides a high degree of stress-strain coupling between the lower extremities of the tines with little adverse inertial mass affect involved. However, because the bending of the tines tends to occur over the entire throat area 6a a significant longitudinal component of inertial force is transmitted to the base 8a and any mounting means which may be associated therewith. This longitudinal force occurs at fork frequency (the vibrational frequency of the fork when struck) because the center of the mass of the fork is not in line with the center of tine oscillation. Again unbalance energy produced by this fork is transmitted to the base 8a and any associated mounting means therefore which may be of material exhibiting substantial molecular frictional loss so as to produce a rapid deterioration or reduction of the fork Q with increasing unbalance between the self-resonant frequencies of the tine members.

Turning now to FIG. 3, one form of flat tuning fork constructed in accordance with the present invention is illustrated and generally indicated by the reference numeral 10. The fork shown may be fabricated by stamping, chemical milling, etching or electroforming methods by means of which a relatively thin strip of the material is removed therefrom to form a first tine 14 which is surrounded by another tine 12 made up of spaced apart side arms 12a and 12b with a connecting link 16 therebetween, both tines extending from the generally rectangular base section 22 for vibration relative thereto generally about the line 13.

In even more particularity, and as shown in FIG. 3, the strip of material removed from the strip is U-shaped with the base of the resulting U-shaped aperture generally indicated at 15 being adjacent the upper edge of the strip. The resulting con figuration of this strip therefor comprises a rectangular base member 22 from which there extends for vibration relative thereto a first rectangular reed member 14 surrounded by a U- shaped member 12 with the outer extremities of the two leg sections of the U-member in turn joining the base member along line 13.

In accordance with the present invention and as is generally shown in FIG. 3, it is desirable to make the width of the center tine 14 equal to the aggregate of the widths of the two leg sections of the surrounding U shown at 12a and 12b. It can be seen that the two leg sections and 12b are effectively linked at the ends thereof which are closest to the base of the U by a member 16. This tends to insure that the two leg sections 12a and 12b move in synchronization with respect to one another. However, it will be readily apparent that the existence of the member 16 connecting the two leg sections of necessity makes the tine 12 longer than the center tine l4 and consequently, it will have a self-resonant frequency which is lower than the self-resonant frequency of tine 14. This fact results in an unbalance of the tines which in accordance with the present invention may be compensated for in a number of ways. For example, a wei'ghtl8 may be added near the tip of the center tine 14 to lower its resonant frequency to that of the outer tine 12. On the other hand, an aperture 20 may be placed near the root of the center tine, as shown, to increase the compliance of its coupling to the base member 22 and hence lower its resonant frequency to that of the outer tine 12.

As in prior art tuning forks, it is desirable to fabricate flat forks of the type contemplated by the present invention from a low molecular loss metal such as Ni Span C referred to above. Alternatively, where it is desired to provide a tuning fork whose resonant frequency remains substantially constant over a wide range of ambient temperature changes, the tuning fork of FIG. 3 and others shown in the drawings and discussed hereinafter, may be made from a sheet of laminated bimetal. Preferably, the material for one of the laminations is chosen from a class of high nickel content alloys which have a positive temperature coefficient of elasticity and the other laminant material is chosen to have a negative temperature coefficient of elasticity. Though these materials should have opposite signs of temperature coefficient, they need not have the same value of coefiicient since they may be compensated for in the choice of the relative thickness of each layer of the laminant. For example, one might use a carpenter 37-FM alloy for one laminant and an ASTME 1095 mild steel for the other laminant. The 37-FM alloy has a positive coefficient of approximately 30 p.p.m. per degree centigrade, while the 1095 steel has a negative coefficient of 300 p.p.m. per degree centigrade In this instance one would make the laminant of the 1095 steel to be only 10 percent of the aggregate thickness of the entire sheet, thus achieving a very near balance or very low net temperature coefficient of elasticity.

Of particular importance in forks made in accordance with the present invention is the part that the base plays in acting to conserve energy transmitted to it as a result of any frequency unbalance between the tines. In accordance with the invention, the free resonance of the base member 22, i.e., the selfresonant frequency thereof when acting as a reed (in the presumed absence of the attached tines l2 and 14) is so related to the resonant frequency of the tuning fork as a whole that it exhibits an independent compliance which is much higher than that of the prior art fork shown in FIG. 1 and much lower than that of the compliance of the tines 12 and 14 which extend from its base 22. Preferably, the resonant frequency of the base as it is mounted to whatever base support means that may be employed should have a resonant frequency which is lower than 2% times the resonant frequency of the fork itself. As a consequence, if there is any unbalanced energy transmitted to the base, the base tends to store the energy during the half periods of the applied vibrational force.

Owing to the low molecular frictional losses in the base material and its low compliance relative to the fork tines, any unbalanced energy transmitted to the base during one halfcycle of operation will be effectively returned to the fork during the succeeding half-cycle of operation. This is in contrast to the situation where the resonant frequency of the base is lower than that of the fork under which circumstances the stored energy produced by unbalance would tend to be returned to the fork at times bucking rather than aiding the movement of the fork tines.

Turning to FIGS. 4A and 48 there is here illustrated the desired mode of vibration of a tuning fork made in accordance with the present invention as by way of example of a form depicted in FIG. 3. This mode of vibration, as in the case of conventional tuning forks, is referred to as the fork-mode and it will be seen that the tines 12 and 14 at any given instant during their motion move in opposite directions with respect to oneanother.

In FIGS. 5A, 5B and 5C here are shown variations of tine geometry which permit establishing the same self-resonant frequency for each tine of a given tuning fork. For example, in FIG. 5A the center tine 14 is shown to be tapered in its width such that it is narrower at its root 14A than it is at its tip 143 thus tending to lower its resonant frequency by increasing the compliance of its interconnection with the base member 22. By properly establishing the extent of this taper the self-resonant frequency of tine 14 can be made equal to the self-resonant frequency of tine 12. v

In the arrangement of FIG. 5B the two legs of the tine 12 are tapered such that they are more narrow at the tips 12' thereof that at their base 12" thus providing less mass at the tips so as to raise the resonant frequency of the outer tine 12 to match that of the center tine 14.

The arrangement shown in FIG. 5C is still another variation of tine configuration which may be employed to reduce the self-resonant frequency of tine 12. Here it can be seen that the end section 16 of the tine 12 is folded back such that the effective length of the tine will be reduced and its self-resonant frequency raised to equal that of the center tine 14. Still in another form (not shown) part or all of the section 16 coupling the two leg sections of the tine 12 may be omitted. In this latter form the resonant frequency of each leg member of the tine 12 must be made substantially equal.

It will be noted that the various means shown in FIGS. 5A, 5B and 5C of achieving a balance between the self-resonant frequencies of the inner and outer tines result in the two tines having different radii of gyration to a certain extent. This effect, however, is not serious. Experimentalforks fashioned in the ways shown herein at resonant frequencies in the 200- to 400-cycle range, have had Qs" in the order of 2,400 to 3,500 thus indicating that a good balance between the tines was achieved such that very little unbalance energy was transmitted to the base 22.

In general the basic frequency of tuning forks constructed in accordance with the present invention may be controlled through the selections of both the thickness of the material used and the compliance thereof as well as by adjusting the lengths of the inner and outer tines. Although the configurations shown in FIG. 3, FIG. 5A, FIG. 5B and FIG. 5C have been found well suited to the fabrication of low-frequency forks (400 Hertz and below) it is been found that similar configurations can be used for medium frequency forks (400 to 20,000 I-Iertz).

Because of the-flat surface existing in and around the base of the tine members, the fork configuration of the present invention is eminently suited for the mounting of piezoelectric crystal elements which may be used for driving and sensing the vibration of such tuning forks. Likewise, aswill be discussed more fully hereinafter, the flat surfaces of tuning forks made in accordance with the present invention are also suited for cooperation with electromagnetic drive and sensing transducers.

Consideration will now be given to various ways in which a tuning fork made in accordance with the present invention can be utilized as the frequency controlling element in the feedback loop of an electromechanical oscillator so as to'provide a driving signal time base suitable for operating an electromechanical clock whose timing accuracy is dependent upon the frequency of the driving signal.

Turning now to FIG. 6, a metallic tuning fork made in accordance with the present invention is generally indicated by the reference numeral 24. Piezoelectric transducers 26 and 28 are affixed to the base 46 of the fork as shown. The piezoelectric elements may be bonded to the fork by various means which are well known in the art. One method of bonding is through the use of an epoxy which cures to a hard or rigid state and which does not soften appreciably over the ambient operating temperature range specified for the fork. When mixing the epoxy, one may add approximately 10 percent by weight of fine nickel powder. The mixture of the epoxy and the fine nickel powder is then applied to the metallized surface of the piezoelectric element and/or the base of the fork at the locations indicated. The piezoelectric element is then placed in the proper position and held in place during curing by applied pressure. The pressure assures that the nickel particles make electrical contact between the metal of the fork and the metallic surface film of the piezoelectric element. Piezoelectric elements may also be bonded to the tuning fork by various soldering techniques provided that the soldering temperature does not reach the Curie temperature of the piezoelectric material. I

Continuing with the description of the arrangement shown in FIG. 6 the piezoelectric element or transducer 28 is positioned near the base ofthe outer tine 24' of the fork 24 to act as apickup or sensing transducer providing an output signal at terminal 28 at the fork frequency. The pickup signal is then applied through resistor 36 to the input of an amplifier 34 the output of which is connected via circuit path 34' to another piezoelectric transducer 26 positioned at the base of the inner tine 25 to excite the tine into vibration. By way of illustration the amplifier 34 is shown powered by a voltage source B. The electrical polarity of the two piezoelectric transducers 26 and 28 and the phase characteristics or polarity of signal amplification provided by theamplifier 34 is such that the fork 24 is driven into sustained vibration at the fork frequency with the tine 24' alwaysmoving in a direction opposite to the tine 25. Thus when tine 24' is moving in the direction of arrow 27 the inner tine 25 is moving in the direction of arrow 27' in accordance with the basic principles of fork-mode" vibration. The output of the amplifier 34 may be in turn applied to the input of an amplifier 35 (also shown powered by the voltage source B) to drive an electric clock such as indicated generally at 39. In order to produce a sustained fork-mode oscillation ator around'400 Hertz the fork 24 may be made. from a strip of Ni Span C metal 0.01 inch in thicknesswith the width of the fork at its base being substantially 0.5 inch and the height of the strip being approximately 1.25 inches. The width of the center tine 25 may be in the order of 0.25 inch with the width of the U-shaped aperture approximately 0.05 inch.

Under many conditions the fork 24 may be subject to spurious vibrational forces as for example of the type which would be encountered if the fork were used in a driving signal time base within a clock mounted in an automobile. In practice these spurious vibrational forces will tend to cause the entire tuning fork 24 to vibrate in a reed-mode, i.e., the outer tine 24' and the inner tine 25 will tend to vibrate with their motion in the same directions simultaneously. That is, tine 24 and tine 25 will in one period of time tend to move in the direction of arrow 27 and thereafter reverse their motion and both move in the direction indicated by arrow 27'. The effect is similar to that of a simple reed vibrating about its base such as is also shown at 30 in FIG. 6 the base 46' thereof also being fastened to some mounting means (not shown). This reedmode type of vibration produces an electrical component in the signal developed by the pickup transducer 28 which is out of phase with the signal produced by a normal fork-mode type of vibration and if the reed-mode component is of sufficient magnitude it can cause the electromechanical oscillator circuit as a whole (comprising the fork 24 and the amplifier 34) to stop oscillating.

To overcome the effects of such spurious vibration the present invention contemplates employing a reed such asis indicated at 30 to which is fastened a piezoelectric pickup transducer 32 to develop a reed-mode signal of opposite polarity to the reed-mode component produced in the output signal of the piezoelectric transducer 28 affixed to the tuning fork 24. In accordance with the present invention the reed 30 is mounted in substantially the same plane as the tuning fork 24 and the resonant frequency of the reed is made equal to the frequency at which the tuning fork 24 tends to vibrate in a reed mode. The signal produced by the piezoelectric transducer 32 being of opposite polarity to that produced by the transducer 28 is coupled through resistor 38 to the input of amplifier 34 so as to cancel the reed-mode component of the electrical signal produced by the transducer 28.

The degree of cancellation which can be effected with the arrangement shown in FIG. 6 may be adjusted by controlling the relative resistance values of resistors 36 and 38 or by adjusting the relative areas of the crystal transducers 28 and 32 or by controlling the relative sensitivities of the two crystal transducers or by adjusting the location of the crystal transducer 32 on its reed member 30. Although the transducers 28 and 32 are shown to be of opposite polarity it is to be noted that they could be mounted on their respective fork and reed members so as to produce like polarity reed-mode signals. With such an arrangement the polarity of the signal produced by the crystal 32 could be inverted by wellknown means prior to its mixing with the signal produced by the crystal 28.

FIGS. 7 and 8 show another embodiment of the present invention in which the pickup crystal or transducer for sensing and developing a cancelling reed-mode signal is not mounted on a separate reed member but is mounted on the tuning fork itself. In FIG. 7 there is shown a tuning fork 40 constructed in accordance with the principles of the present invention with a center tine 42 and an outer tine comprising leg member 44. Adjacent to the lower extremities of the tines and at the upper part of the base portion 46 are mounted three piezoelectric crystal transducers all along a given line axis. A drive crystal 48 is mounted adjacent the right-hand edge of the fork under the right-hand leg member 44 and two pickup crystals 50 and 52 are mounted under the center tine 42 and the left-hand tine leg member 44 respectively as shown. The dotted line configuration shows the tines to be vibrating in the fork-mode, i.e.,

the center tine is moving in opposition to the outer tine. As the outer tine moves such that the pickup crystal 52 goes into tension, the center tine 42 moves in the opposite direction and pickup crystal 50 goes into compression. Since pickup transducers 50 and 52 are of opposite polarities, fork-mode vibration will cause the respective signals produced thereby to be of the same polarity and in phase. The transducers 50 and 52 may then be connected together in parallel, by circuit path means 53 and thence to the input terminal of an amplifier 53'. The output of amplifier 53' is then connected to the drive transducer 48 via circuit path 53". An output signal from the overall oscillatory time base arrangement for application in driving a clock may be taken off at terminal 54. Thus in this arrangement both the pickup transducers 50 and 52 together function as a normal pickup transducer for fork-mode vibrations. Since these transducers are connected in parallel they together provide a pickup source impedance or reactance which is half of that of a single transducer of the same size. This allows the associated circuits to represent a less significant load on the pickup transducers and effect a higher system gain.

In FIG. 8 the reed-mode type of vibration is illustrated by the dotted lines associated with the fork 44, i.e., where both tines are moving in the same direction at the same time. In this case it will be noted that when the transducer 52 provides a negative polarity the transducer 50 provides a positive polarity in response to the reed motion. That is when the fork bends as shown by the dotted lines both pickups 50 and 52 go into tension and provide opposite polarity voltages. Since both are located in similar stress points along the fork, and are of substantially equal size, the generated voltages will be equal in amplitude, hence effecting cancellation of any signals produced by reed-mode vibration. It is in this manner that the effects of any reed-mode vibrational influences will be cancelled out when the tuning fork is operating in a normal forkmode as shown in FIG. 7.

A significant advantage of the embodiment of the inventions shown in FIGS. 7 and 8 is that either pickup 50 or 52 is located remotely enough from the drive crystal 48 that the strain coupling between the pickup 48 and the pickups 50 and 52 is minimized. Strain coupling may be defined as that coupling between the drive and pickup crystals due to strains in the fork material which are not related to the tine movement. The presence of excessive strain coupling between the drive and pickup transducers can result in a feed-back path around the electronic circuit associated with the transducers which allows the circuit to oscillate at some spurious frequency not related to the fork frequency. It should be understood, however, that the arrangement of crystal transducers on the base of the fork may be varied in accordance with fork geometry such as to minimize strain coupling therebetween. It is contemplated, therefore, that the transducers need not necessarily be in alignment as shown but may be positioned in a staggered pattern or in some cases may be placed on 0pposite sides of the fork with the fork material therebetween. For example, either pickup crystal transducer 50 or 52 could be placed on the reverse side of the fork from the drive crystal 48 and polarized to provide the above-stated functions or both pickup crystals could be placed on the reverse side of the fork opposite the drive crystal.

Turning now to FIG. 9 there is shown still another tuning fork configuration contemplated by the present invention along with drive and pickup transducers for enforcing forkmode vibration at the same time cancelling reed-mode vibrational effects. The configuration of the fork 60 shown in FIG. 9 comprises an outer tine member 62 comprising legs 62 and 62" extending from a base section 64. In order to bring the self-resonant frequency of the outer tine 62 to the same value as the self-resonant frequency of the inner tine member 66 the width of the leg members 62 and 62 is reduced at their upper extremities as shown at 63 and 63'. By reducing the width of the outer tine leg members at these positions the mass of the outer tine member is of course reduced tending to lower its self-resonant frequency. Apertures 65 and 65' are also provided just above the upper portion of the fork base 64 and it can be seen that the apertures 64 and 65' relieve the center tine member 66 of more material than they do the legs 62' and 62" of the outer tine member 62. This tends to increase the compliance of the coupling between the center tine member 66 to the base 64 thus reducing its self-resonant frequency at the same time reducing further the self-resonant frequency of the outer tine member 62 so as to permit the realization of very low fork frequencies for a given set of fork dimensions.

By way of example a 400 Hertz tuning fork of the type illustrated in FIG. 9 can be made of a Ni Span C strip 0.01 inch in thickness, 0.460 inch in width and 1.275 inches in height. The width of the U-shaped portion of the aperture therein is made approximately 0.06 inch wide with the elongated apertures 68 and 68' being 0.362 inch long and 0.166 inch wide with the upper and lower extremities thereof being defined by the circumference of a circle having a radius of 0.083 inches. The center of axis of the elongated aperture is coincident with the center longitudinal axis of the U-shaped aperture. The lower extremity of each aperture is 0.503 inch from the lower extremity of the base 64. The outer tine leg members 62 and 62' are made 0.1 inch wide with the reduced portions 63 and 63' thereof being made 0.03 inch wide. The height of the leg portions 62 and 62" above the lower extremity of the base 64 is 1.005 inches while the reduced leg portions 63and 63 extend 0.270 inch above the upper extremities of the leg portions 62 and 62". The upper extremity of the center tine is 1.215 inches from the lower extremity of the base 64.

Further, with respect to the embodiments shown in FIG. 9,

there is shown two pickup transducers 70 and 72 correspond ing to pickup transducers 50 and 52 shown in FIGS. 7 and 8 with a drive transducer for the fork being shown at 74. It will be noted that in accordance with this embodiment of the present invention the driving transducer 74 is not mounted to the base 64 of the tuning fork over the entire surface area of the transducer as shown in previous embodiments but is coupled to the tuning fork base by two coupling members generally indicated at 76 and 78 along a line parallel to the longitudinal axis of the outer tine member 62. By this mechanism adverse lateral strain coupling to the pickup transducers 70 and 72 is minimized.

As indicated hereinbefore, the flat configuration of tuning forks made in accordance with the present invention permits such forks to being easily driven and their vibrations sensed by electromagnetic as well as piezoelectric transducing means. The arrangement in FIG. is illustrative of this feature of advantage. The tuning fork 82 shown therein has an inner tine 84 and an outer tine 86. Associated with the outer tine 86 is a magnetic pickup transducer 88 having a magnetized core about which a wire coil is wound for sensing the motion of the outer tine 86. This pickup transducer 88 is connected in series opposing with another like magnetic pickup transducer 90 which is associated with the center tine 84. The series-opposing configuration of the two transducers is in turn connected between ground and the input of amplifier 92. The output of amplifier 92 is then connected via switch 94 to a driving magnetic transducer 96 the magnetic field of which drives the center tine 84. The. polarity or phase characteristics of the am. plifier 92 taken in combination with the winding polarity of drive transducer 96 is such that in the normal fork-mode of vibration motion of outer tine 86 toward the transducer 88 along with the motion of inner tine 84 away from transducer 90 produces an input signal to the amplifier 92 which when amplified thereby and applied to the drive transducer 96 causes the magnetic field produced thereby to repell the center tine 84. It will be seen that any reed-type vibration will cause identical signals of the same polarity to be indiced in and appear across the windings of pickup transducers 88 and 90. As these transducers are connected in series-opposing relationship the reed-mode signals will cancel each other so that the input of the amplifier 92 will receive only signal components corresponding to fork-mode operation. The transducers 96 may be of the same construction as transducers 88 and 90.

Still referring to the embodiment of FIG. 10, it can be seen that alternatively the switch 94 may be positioned as shown by the dotted lines 98 to drive a piezoelectric driving transducer means comprising elements 100, 100a and 10% so as to produce fork vibration. In this latter configuration, of course, there will be an absolute absence of adverse strain coupling between the sensing transducers and the drive transducers. As previously shown hereinbefore, the output of the amplifier 92 may be coupled to the input of a driving amplifier 108 to power an alternating current clock generally indicated by the rectangle 1 l0.

In still another form of the present invention such as shown in Fig. 11 a single magnetic driving transducer may be associated with either the inner or outer tines of a flat tuning fork made in accordance with the present invention. By way of example, the tuning fork generally indicated at 82 has a magnetic driving transducer 112 associated with the center tine 84'. Tine vibration in this arrangement is sensed by pickup transducers 114 and 116 in the same manner practiced in the embodiment shown in FIG. 7. The output of the pickup transducers 114 and 116 which are of opposite polarity (for purposes of reed-mode signal cancellation) are connected in parallel and applied to the amplifier 118 which is in turn connected in driving relationship to the driving transducer 1 12. In this arrangement when the driving transducer 112 produces a field repelling the center tine 84 the transducers 114 and 1 16' will produce a signal of a polarity which when amplified by the amplifier 118 will drive the transducer 112 to repel even further the center tine 84'. In this embodiment of the present invention it is seen again that there is an absolute absence of strain coupling between the drive and pickup transducers. The output of the amplifier 118 may be coupled to a driving amplifier such as 120 suitable for driving into operation an alternating current clock 122.

The foregoing description of the present invention as it relates to the design, method of manufacture and use of the improved tuning' forks contemplated thereby is only illustrative of specific forms which the present invention may take. Still other modifications and variations will suggest themselves to persons skilled in the art. It is intended therefore that the foregoing detailed description be considered as exemplary only and that the scope of the invention be ascertained from the following claims.

That which is claimed is:

1. A planar vibrator plate having a generally rectangular outline, wherein one end of said plate constitutes a mounting base, and a U-shaped aperture in said plate extending lengthwise thereof defines a first vibrator element lying within said aperture and a second vibrator element surrounding said aperture, the side legs of said U-shaped aperture being of truncated triangular shape, whereby the cross section of at least one of said vibrator elements varies uniformly with respect to distance from said base.

2. A planar vibrator plate having a generally rectangular outline, wherein one end of said plate constitutes a mounting base, and a U-shaped aperture in said plate extending lengthwise thereof defines 'a first vibrator element lying within said aperture and a second vibrator element surrounding said aperture, the side legs of said U-shaped aperture terminating in openings wider than the legs of said U-shaped aperture, elongated in the direction of the longer dimension of said rectangular plate, and of semicircular outline at their ends.

3. A planar vibrator plate according to claim 2, wherein the center axis of each opening is coincident with the center axis of the leg of said U-shaped aperture terminating in said openmg.

4. A vibrator fork structure comprising a thin, flat, compliant, rectangular sheet of uniform thickness having a U- shaped aperture therein extending longitudinally of said sheet, the portion of said sheet surrounded by said U-shaped aperture defining a first tine member, the remainder of said sheet surrounding said U-shaped aperture defining a second tine member, and the portion of said'sheet integral with both said tine members defining a base.

5. A vibrator fork structure in accordance with claim 4 wherein said first tine member is provided with an aperture in a portion thereof adjacent the to increase the compliance of the coupling between said first tine member and said base whereby the self-resonant frequency of said first tine member is reduced relative to the self-resonant frequency thereof in the absence of said aperture.

6. Apparatus according to claim 4 wherein said vibrator fork structure is formed of a thin strip of laminated bimetallic material having a low net temperature coefficient of elasticity.

7. A vibrator fork structure according to claim 4 wherein portions on each side of said first tine member and portions of both sections of said second tine member are removed at positions thereon adjacent said base to increase the compliance of the coupling between each tine member and said base whereby the self-resonant frequencies of both tine members are reduced relative to the self-resonant frequencies which would otherwise be assumed thereby.

8. A vibrator fork according to claim 7 wherein the portion removed from said first tine member is greater than that removed from said second tine member whereby the resulting self-resonant frequency of said first tine member is made substantially equal to the resulting self-resonant frequency of said second tine member.

II I I I l 

1. A planar vibrator plate having a generally rectangular outline, wherein one end of said plate constitutes a mounting base, and a U-shaped aperture in said plate extending lengthwise thereof defines a first vibrator element lying within said aperture and a second vibrator element surrounding said aperture, the side legs of said U-shaped aperture being of truncated triangular shape, whereby the cross section of at least one of said vibrator elements varies uniformly with respect to distance from said base.
 2. A planar vibrator plate having a generally rectangular outline, wherein one end of said plate constitutes a mounting base, and a U-shaped aperture in said plate extending lengthwise thereof defines a first vibrator element lying within said aperture and a second vibrator element surrounding said aperture, the side legs of said U-shaped aperture terminating in openings wider than the legs of said U-shaped aperture, elongated in the direction of the longer dimension of sAid rectangular plate, and of semicircular outline at their ends.
 3. A planar vibrator plate according to claim 2, wherein the center axis of each opening is coincident with the center axis of the leg of said U-shaped aperture terminating in said opening.
 4. A vibrator fork structure comprising a thin, flat, compliant, rectangular sheet of uniform thickness having a U-shaped aperture therein extending longitudinally of said sheet, the portion of said sheet surrounded by said U-shaped aperture defining a first tine member, the remainder of said sheet surrounding said U-shaped aperture defining a second tine member, and the portion of said sheet integral with both said tine members defining a base.
 5. A vibrator fork structure in accordance with claim 4 wherein said first tine member is provided with an aperture in a portion thereof adjacent the base to increase the compliance of the coupling between said first tine member and said base whereby the self-resonant frequency of said first tine member is reduced relative to the self-resonant frequency thereof in the absence of said aperture.
 6. Apparatus according to claim 4 wherein said vibrator fork structure is formed of a thin strip of laminated bimetallic material having a low net temperature coefficient of elasticity.
 7. A vibrator fork structure according to claim 4 wherein portions on each side of said first tine member and portions of both sections of said second tine member are removed at positions thereon adjacent said base to increase the compliance of the coupling between each tine member and said base whereby the self-resonant frequencies of both tine members are reduced relative to the self-resonant frequencies which would otherwise be assumed thereby.
 8. A vibrator fork according to claim 7 wherein the portion removed from said first tine member is greater than that removed from said second tine member whereby the resulting self-resonant frequency of said first tine member is made substantially equal to the resulting self-resonant frequency of said second tine member. 